Author: Tanner Marshall, MS
Editor: Rishi Desai, MD, MPH, Tanner Marshall, MS
The atria are the heart’s upper chambers, and the ventricles are the lower chambers, reentrant tachycardias are fast heart rates caused by electrical signals that loop back on themselves.
Normally, an electrical signal starts at the sinoatrial or SA node in the right atrium, it then propagates out through both atria, including bachmann’s bundle in the left atrium, then contracts both atria, it’s then is delayed just a little bit as it goes through the atrioventricular or AV node, before it passes through the Bundle of His and on to the Purkinje fibers of the left and right ventricles, causing them to contract as well.
Usually, the only place where a signal can go from the atria to the ventricles is at the AV node, and once that signal gets to the purkinje fibers, it stops, and the heart tissue waits for another signal from the SA node. With an atrioventricular reentrant tachycardia, or AVRT, the electrical signal actually uses a separate accessory pathway to get back up from the ventricles to the atria, causing the atria to contract before the SA node sends out another signal. The signal then moves back down the AV node, to the ventricles and purkinje fibers and contracts the ventricles, as well as goes back up that accessory pathway, and this cycle repeats, which is why AVRT can result in rates as high as 200-300 bpm. This type of tachycardia is known as a supraventricular tachycardia, because the signal causing the fast rate originates above the ventricles. The most common type of AVRT is Wolff-Parkinson-White syndrome, where the accessory pathway is called the Bundle of Kent. This type of reentry is known as an anatomical reentrant circuit, because the accessory pathway is a fixed anatomically defined pathway.
Another type of reentrant circuit, though, is atrioventricular nodal reentrant tachycardia, or AVNRT. AVNRT, just like AVRT, is a type of supraventricular tachycardia, but with AVNRT it’s in or near the AV node, which just like before contracts the ventricle and the atria every time it goes around.
Specifically, there are two separate electrical pathways that make up this loop, and one of these pathways has heart tissue that has slow electrical conduction, and is called the alpha pathway, and the other has fast conduction—and it’s called the beta pathway. Not only that, though, the alpha pathway has a short refractory period, which is the time it takes to be able to conduct another signal. The beta pathway, on the other hand, has a long refractory period. Once you have all those things you’ve got yourself a recipe for AVNRT.
So now let’s say a signal comes down from the SA node in the right atrium, the signal goes down the fast pathway and reaches the other end before the slow pathway. Then it splits to travel down to the ventricles, as well as up the alpha pathway where it meets the slow signal and they both cancel each other out.
At this point, both enter into their refractory period, except the alpha pathway’s is shorter right, so it comes out of refractory sooner and is ready for another signal, while the beta pathway’s still in refractory. So if another signal comes by, it’ll start down the slow pathway, but since the fast is still in refractory, it’ll be blocked.
At some point while the signal’s going down the alpha side, that beta side will come out of refractory, and then it’ll be ready to go. So now as the signal exits the alpha pathway to the ventricles and enters the refractory period.
It also travels up the beta pathway, and by the time it reaches the alpha pathway again, that pathway’s out of refractory! So now you have this reentrant loop set up that keeps going and going, and every time around it causes both an atrial and a ventricular contraction, which again just like AVRT, leads to high ventricular rates. This particular setup, where it goes down the slow pathway, or anterograde, and up the fast pathway, or retrograde, is called slow-fast AVNRT, or typical AVNRT.
It’s also possible to have the opposite, though, called fast-slow AVNRT or atypical AVNRT, which as the name suggests is a lot less common than slow-fast AVNRT. With this type the pathway goes down the fast pathway or anterograde, and up the slow pathway or retrograde.
The tachycardia with AVRT and AVNRT typically doesn’t last very long, and both are rarely life-threatening. They can, though, both produce symptoms like palpitations, shortness of breath, and feelings of dizziness, as well as syncope or fainting in rare cases.
With an ECG, the p wave is the signal from atrial contraction and the QRS is the signal from ventricular contraction, on an ECG with AVNRT, the P waves might not be visible since the signal’s getting to the atria and ventricles at almost the same time, so the p wave starts essentially where the QRS starts and when you add them together, the p wave can get buried under the QRS complex. With AVRT, the p waves might or might not be buried, depending on where the accessory pathway is located.
The definitive treatment for either AVRT or AVNRT, is radio catheter ablation, essentially destruction of the accessory pathway with AVRT, and destruction of the slow or alpha pathway with AVNRT. For AVNRT, sometimes people can use vagal maneuvers as well, which are ways to activate the vagus nerve, which tends to block the AV node temporarily, therefore potentially stopping the episode. Some methods include a carotid sinus massage, as well as a valsalva maneuver—which is forced exhalation against a closed airway. Also, they may use medications to slow AV node conduction. In rare cases where other treatments haven’t been effective, though, they might need cardioversion.